Merry go round uses the theory of
centripetal force
From the above diagram of Merry go round it is clear that it works on the principle of centripetal force acting on the connected masses with the help of a metal rod or chain. To make an object move in anything other than a straight line, you have to exert a force on it. A force is required for an object to move in a circle, as is the case with a merry-go-round.